1. Field of the Invention
The present invention relates to a magnetic resonance imaging (hereinafter referred to as MRI) apparatus. Particularly, it relates to an MRI apparatus and a method suitable for obtaining a high-quality image by maintaining a magnetic field at a high homogeneous value.
2. Description of Related Art
In medical organizations, there is widely used MRI apparatuses that obtain a tomographic image of a human body by utilizing the nuclear magnetic resonance (NMR) phenomenon. Such apparatuses require a magnet that generates a magnetic field of uniform intensity in a space where the part of an object (e.g., a patient) to be examined is disposed in order to accurately image the internal structure of the part. Ideally, a uniform magnetic field space would be obtained within a solenoid coil of infinite length. Therefore, many of the magnets used in MRI apparatuses have slender cylindrical solenoid coils and shimming mechanisms that improve the uniformity of the magnetic fields.
In contrast to the MRI apparatus having a slender space for disposing a person (patient) to be examined, there has also been developed an MRI apparatus which employs a magnet structure with a short length or an open magnet structure having a wide opening. This MRI apparatus helps the patient feel more relaxed by relieving from the sense of confinement and makes it possible to apply an interventional method where the operator provides treatment during the examination using the MRI.
The magnet of short length or the magnet having the open structure uses an iron yoke to establish a magnetic circuit. The problem of this structure is that when the iron yoke thermally expands, the intensity and uniformity of the magnetic field change depending on the temperature.
In order to solve this problem, a method was developed for controlling the magnet to keep its temperature constant. One example of this technique is disclosed in Japanese Patent No. 2566410.
On the other hand, two shimming methods for improving the uniformity of the static magnetic field are available. One is a passive shimming that disposes a magnetic piece to cancel the non-uniformity of the static magnetic field. The other is active shimming that changes the current passed through a shim coil in response to uneven variation in the static magnetic field. One proposed active shimming ascertains the level of magnetic field non-uniformity by analyzing an NMR signal detected from the examined object, and controls the current passed through the shim coil based on this non-uniformity level. This is disclosed in, for example, U.S. Pat. No. 5,530,352.
The method of controlling the temperature of the magnet itself is an effective method in that it can generate a stable magnetic field regardless of the structure or shape of the magnet. However, this method has the following drawbacks.
(1) In order to keep the magnet at a constant temperature, the magnet temperature should be kept higher than the normal room temperature. For this purpose, the magnet needs to be surrounded by a high-temperature bath. This narrows the space for disposing the object to be examined.
(2) The magnet has large heat capacity and therefore takes a long time to reach a constant temperature. The temperature control circuit must therefore be operated for a considerable time before the magnet is used for an examination. In actual practice, the magnet is kept applied with current continuously. When a power failure occurs, the apparatus cannot be used until the magnet has recovered a stable temperature level.
(3) It is not possible to cope with a substantial or sudden change in the temperature around the magnet which exceeds the capacity and tracking speed of the magnet temperature control circuit.
Further, in new imaging methods which require high uniformity of the magnetic field (such as imaging in which only a fat signal is suppressed, or echo planar imaging EPI, etc.), it is necessary to control the temperature with higher precision and to maintain the uniformity of the magnetic field in a more stable condition than in earlier methods.
On the other hand, in the method of reducing the magnetic field non-uniformity by using an NMR signal from the examined object, it is necessary to adjust the uniformity of the magnetic field after the object has been disposed in the magnetic field. This lowers the efficiency of the MRI examination. Further, the non-uniformity of the magnetic field is different depending on the part of the patient to be examined and the disposed position of the patient. Therefore, this method involves complex adjustments, and is incapable of efficiently overcoming the non-uniformity due to temperature variations.
An object of the present invention is to provide an MRI apparatus and a method that can stably maintain the uniformity of the magnetic field without being affected by variations in temperature. Another object of the invention is to provide an MRI apparatus that can achieve high uniformity of the magnetic field and that can improve the quality of MRI examination data.
In order to achieve the above objects, according to one aspect of the present invention, there is provided an MRI apparatus which comprises: a static magnetic field generating unit that generates a static magnetic field of a constant magnetic field strength; a gradient magnetic field generating unit that generates a magnetic field strength gradient; a unit that generates a high-frequency magnetic field; a unit that detects a nuclear magnetic resonance signal generated from an object to be examined; and a unit that displays a result of the detection, wherein the MRI apparatus further includes a magnetic field correcting unit that generates an additional magnetic field for making uniform a space distribution of the static magnetic field; a unit that detects a temperature of the static magnetic field generating unit and/or surroundings thereof; and a unit that controls the magnetic field correcting unit based on the temperature detected by the temperature detecting unit.
According to the above aspect, the magnetic field correcting unit is controlled based on a detected temperature. The non-uniformity of the static magnetic field is corrected based on this control. Therefore, it is possible to eliminate non-uniformity of the magnetic field due to temperature variations with good response. Accordingly, it is possible to cope with substantial variations in temperature or sudden changes in temperature. It is also possible to eliminated the need for a high-temperature bath for keeping the static magnetic field generating unit at a constant temperature. Therefore, a large space for disposing the object to be examined can be secured. This makes it possible to improve the efficiency of interventional work.
According to the MRI apparatus of the present invention, the magnetic field correcting unit may include a shim coil and a unit that supplies a current to the shim coil. The control unit controls the current supplied to the shim coil, thereby improving the uniformity of the static magnetic field.
Preferably, the control unit controls the magnetic field correcting unit based on a temperature characteristic of the non-uniform component of the space distribution of the static magnetic field. The temperature characteristic of the non-uniform component can be ascertained in advance. The control unit calculates the non-uniform component at a detected temperature based on the detected temperature and the temperature characteristic, and controls the magnetic field correcting unit so as to generate an additional magnetic field that cancels the component.
Further, according to the MRI apparatus of the present invention, the temperature detecting unit may be disposed near the static magnetic field generating unit and/or in the room where the static magnetic field generating unit is disposed. For example, a thermometer disposed within the room can play the role of the temperature detecting unit. In this case, it is preferable for the thermometer to have a temperature setting unit that supplies the detected temperature to the control unit. The control unit controls the magnetic field correcting unit based on the temperature within the room set by the temperature setting unit. The temperature may be set by the temperature setting unit as a numerical value or a qualitative parameter.
Further, according to a preferred embodiment of the MRI apparatus of the present invention, the temperature detecting unit detects temperatures of at least two positions including the static magnetic field generating unit and/or its surroundings. The temperature characteristic of the non-uniform component of the space distribution of the static magnetic field is obtained in advance for each temperature change at each position. The control unit calculates the non-uniform component at the detected temperature from the temperature detected at each position and the temperature characteristic. The control unit then corrects the magnetic field correcting unit so as to generate an additional magnetic field that cancels total non-uniformity of the respective positions.
When a magnetic field is corrected based on temperatures detected in advance at a plurality of positions, it is possible to cope with a complex and uneven change in the temperature, such as, for example, a local temperature change attributable to an eddy or substantial temperature variations around this position.
According to another embodiment of the MRI apparatus of the present invention, the magnetic field correcting unit generates at least one additional magnetic field of y linear term, z quadratic term and z quartic term, where z is the direction of the static magnetic field and y is a direction orthogonal thereto. The control unit corrects the magnetic field for at least one of the y linear term, z quadratic term and z quartic term based the detected temperature.
According to another aspect of the present invention, there is provided a method for maintaining uniformity of a static magnetic field, which is a method of maintaining uniformity of a static magnetic field generated by a static magnetic field generating unit in an MRI apparatus, by generating an additional magnetic field. This method includes the steps of: obtaining a temperature dependence of a non-uniform component of a space distribution of a static magnetic field; detecting a temperature of the static magnetic field generating unit; and obtaining an intensity of the additional magnetic field based on the detected temperature and the temperature dependence.
According to the method for maintaining uniformity of a static magnetic field, the steps from the temperature detection to the generation of the additional magnetic field may be operated at all times, or may be operated as required or periodically.
The method for maintaining uniformity of a static magnetic field may be performed together with active shimming that corrects error of magnetic field due to a magnetization of the object to be examined. Such a method for maintaining uniformity of a static magnetic field according to the present invention further comprises the steps of: measuring NMR signals generated from an object to be examined; calculating a magnetic field error component attributable to the object using the measured NMR signals; calculating a strength of the additional magnetic field based on the error component attributable to the object; and generating an additional magnetic field having an intensity equal to that of the sum of that obtained based on the detected temperature and the temperature dependence and that calculated based on the error component.